The conventional method for scanning and processing an image is unidirectional, which will results in textures, degrade the random characteristic based on the error diffusion, and thus impact the quality of the output. The method of bidirectional scanning and processing an image can address the problems in a certain extent. In the method of bidirectional scanning and processing, lines of the image is scanned and processed in two directions, alternatively (i.e. from the left to the right for one line and from the right to the left for the next line). Currently, many methods for scanning and processing an image on the basis of the error diffusion are bidirectional.
However, the method of bidirectional scanning and processing needs to operate an image pixel by pixel, which results in a low efficiency. The general method for solving this problem is to improve the hardware. If pixels are scanned one by one in a single direction, a pixel can be processed and output as soon as the pixel is scanned, without needing buffer storage for data of the scanned pixels. However, when a method of bidirectional scanning and processing is used, the results of error operation of the processed pixels should be weightedly added to adjacent unprocessed pixels according to the principle of error diffusion. Some of the unprocessed pixels are in the next line which is un-scanned. Therefore, if pixels are scanned and processed bidirectionally, the data of pixels scanned in two different directions must be stored and then processed one by one. Particularly, in the event of bidirectional scanning and processing, if the current line is scanned from the left to the right, the next line will be scanned from the right to the left. Before the firstly scanned pixel in the next line is processed, errors of last several scanned pixels in the current line are needed to be diffused to the firstly scanned pixel in the next line. Therefore, the value of error diffusion for a pixel in the next line can be obtained only after all pixels in the current line are scanned and processed.
The inventors have disclosed a method for screening in a previously filed Chinese patent application No. 200510068127.8 published on Sep. 14, 2005, entitled “method for frequency-modulation screening using error diffusion based on dual-feedback”, which is incorporated herein by reference. According to the method, an FM-AM mixed screening can be achieved by using a dual-feedback error diffusion technology. However, the method also needs to previously store the data of pixels scanned in two different directions and then process the stored data one by one so as to achieve the bidirectional scanning.